An alkylaromatic polycarboxylic acid, e.g. isophthalic acid (IPA), terephthalic acid (TA), trimellitic acid (TMLA), 2,6-naphthalenedicarboxylic acid (2,6-NDA), and the like is produced and recovered from the product stream of a large-scale facility for the liquid-phase, homogeneously catalyzed oxidation of the appropriate precursor alkylaromatic compounds under pressure (referred to in the art as the "Amoco.RTM. Mid-Century.RTM. process", or "Mid-Century" process, for brevity). The catalytic reaction is carried out with air in the presence of an acetic acid/water mixture which functions as a solvent for the reactants. The process produces a stream of undesired materials as a residue which entrains components of the catalyst. This residue comprises a mixture of oxygen-containing derivatives of the reactants and reaction products including partially oxidized and dealkylated oxidized mixtures of aromatic compounds, tars, and ring-brominated aromatic compounds, much of which residue is complexed with components of the catalyst used, namely Co--Mn--Br, or, Co--Mn--Br--Ce (cerium), or, Co--Mn--Br--Zr (zirconium). The compounds include acetates, bromides and bromoacetates of Co and Mn; a wide array of aromatic and polynuclear carboxylic acids, aromatic and polynuclear aldehydes; aromatic and polynuclear mixed carboxylic acid aldehydes, including ring-brominated aromatic compounds; and, unidentified Co and/or Mn complexes and salts of the anions of acetic acid, hydrobromic acid and any of the afore-mentioned aromatic carboxylic acids.
Though the amount of this residue is relatively small, typically in the range from 0.1 to 25 weight percent of the polycarboxylic acid produced, though usually less than about 10%, the net amount of such residue produced annually in a commercial plant is so large that it is desirable to recover the metal components, specifically the Co and Mn, and the halogen component, namely Br and compounds of Br. Hereafter, for ease and convenience, the term "Br value" refers to either molecular Br, or bromine compounds such as HBr and MnBr.sub.2, or the bromine content or value of the stream.
To date, this residue has been treated in the following main ways: (i) incineration to provide flyash for further processing, namely, to recover its metal content; or, (ii) discharging to a residue pond notwithstanding the loss of the value of the Co, Mn and Br content in the resulting earthy residue, or the adverse environmental impact of doing so; or, (iii) precipitating the metal values as carbonates, treating the remaining organic components and halides by dilution followed by anaerobic digestion and reconstituting catalyst by dissolving metal carbonates in acetic acid; or, (iv) calcining the residue to oxides and utilizing the oxides for other applications.
The term "residue" is used hereinbelow to refer to both plant residue as well as earthy residue, one or the other being referred to specifically when both are not included.
Referring to FIG. 1 there is schematically illustrated the main steps of a currently used commercial process for recovering catalyst from the residue. As described in U.S. Pat. Nos. 4,876,386 and 4,786,621 to Holzhauer et al, the organic matter in the residue is destroyed by incineration while the catalyst components are converted to an ash. This ash is difficult and/or expensive to convert to reusable forms of catalyst for the oxidation of methyl-substituted benzenes.
In greater detail, the residue stream is incinerated in step 2 to produce a mixed metal oxide flyash which is collected in step 3. Since not all the Co and Mn from the residue is transferred into the flyash collected, the remainder is lost in the incinerator's residue discharged to step 6. Collected flyash (from the incinerator in step 2) is washed with water in step 4 to remove the soluble salts and sodium bromide which are discarded (step 5). In the next step 7, the washed ash containing a major proportion (&gt;50%) by weight of Co and Mn is converted to acetates and bromides of Co and Mn by digestion and extraction before being returned to catalyst inventory (step 9). Material not extracted from the washed ash is discarded (step 6). Catalyst is fed from storage (9) to the process (step 10). A portion of the catalyst from step 10 is recycled internally in step 11, being returned to storage of catalyst in step 9 for re-use in the liquid-phase oxidation reactor in step 10 or directly returned to the process, while the desired products of the reaction are separated and sent elsewhere for further processing. A purge stream from step 11 generates the residue stream 1. This residue is then incinerated to start the recovery and re-use process anew. As is evident, some portion of the metal content of the catalyst, typically from 30% to 40%, and depending upon the quality of the flyash and conditions for processing it, as much as 90% of the residue's metal content, is lost from this system, and inevitably all the Br.
In the process just described, the Co and Mn components not lost in step 3 are extracted from the flyash with aqueous acetic acid and by reducing them with hydrazine. This is done by refluxing with a 10% hydrazine solution in aqueous acetic acid. This recovery process results in the loss of a substantial portion of the Co and Mn. Despite the economic incentive (a) to recover substantially both main metals (Co and Mn) and also bromine from the residue, and (b) destroy the waste organic content of the residue, there is no suggestion in the prior art to do so, much less how to do so.
The alternative to incineration and treating flyash, namely discharging to a sludge pond, results over time, in an earthy residue which represents a large recoverable accumulation of main metals Co and Mn, and the halogen Br in the form of bromine compounds. This accumulation, though arguably a non-hazardous waste, concurrently represents a valuable resource and, if recovered, would lead to restoration of a safe environment. Discharging these wastes to a pond often leads to contamination with earthy components, such as silica, alumina, clay and the like. I know of no prior art technology which can process both organic residues, namely, this earthy residue, and the organic residue which is the source of the earthy residue.
Treatment of residues with sodium carbonate to recover the metals as carbonates, followed by dilution and anaerobic digestion is said to be inefficient, (with metal recoveries in practice being approximately 50%) and expensive, with costs approximating those of incineration. Further, halides are not recovered in this process. The anaerobic digestion of the organic compounds is also inefficient, requiring careful control of conditions to maintain an active biological culture and in the end still producing a sludge requiring disposal. Treatment of residues by calcination and using the oxide products of calcination in other applications requires a large central facility for efficient operation, fails to recover halides; and, by not returning catalyst to the manufacturer, forces the manufacturer to cope with a widely fluctuating market supply of Co and Mn, which in turn causes their prices to be highly unstable.
The process of my invention accomplishes the recovery from both residues. Destruction of organic residues and segregation of their components is effected by charging either of these streams to an appropriate pyrometallurgical treatment system, in combination with the required amount of an oxygen containing gas. By "an appropriate pyrometallurgical treatment system", I refer to a pyrometallurgical system in which the metal components are rendered into a reduced molten state. Such systems include those in which: (i) organic residue is directly introduced into a molten metal bath as described in detail in the parent application; or, (ii) an electric or plasma arc or torch, either directly or indirectly heats the organic residue, resulting in a pool of molten metal; or, (iii) an induction heating means heats the organic residue, resulting in a pool of molten metal; or, (iv) the organic residue is charged to a pool of molten salt or molten glass and the molten metal separated and collected from the bottom of the system.
Relevant prior art, teaching pyrometallurgical treatment systems which may be used for the destruction of hazardous wastes generally, is summarized as follows:
Processes for the destruction of organic waste in a bath of molten metal in the presence of oxygen, require maintaining a temperature high enough to convert the residue to oxides of carbon and to convert the metal component to a form which will dissolve in the melt. Such a melt having a viscosity no greater than 10 centipoise has been used to destroy toxic chemicals by injecting a greater than stoichiometric amount of oxygen into organic waste fed to the bath, as disclosed in U.S. Pat. No. 4,574,714. Additional references teaching molten baths for the purpose of destroying toxic chemicals are discussed in the '194 application, the disclosure of which is incorporated by reference thereto as if fully set forth herein.
The parent '194 application specifically teaches how a molten metal bath may be used to treat the residue from a Mid-Century plant and capture the Co and Mn metal values in a molten bath of essentially the same metals. Similar results may be acheived by the use of electric and plasma arc systems, of which numerous examples are known.
U.S. Pat. No. 4,431,612 to Bell et al describes the use of a high current DC electric arc, which includes a sump containing a molten bath, which can be a molten metal, and the accumulation of metallic elements from mixed chlorinated wastes (PCB's and PCB contaminated materials) charged to the system in the molten metal bath.
U.S. Pat. No. 5,534,659 to Springer et al describes the use of a plasma arc for treating organic and inorganic wastes and isolating molten metal and slag therefrom, and adding steam to the system to ensure the conversion of all carbon containing components to carbon monoxide and carbon dioxide.
In Proceedings of the International Symposium on Environmental Technologies: Plasma systems and Applications, Oct. 8-11, 1995, Atlanta, Ga., pp 251, Retech, Inc. describes an apparatus for the treatment of wastes using a plasma torch in a centrifugal chamber, with the discharge of molten metal through a center hole, and subsequent use of atomization to render the metal in powder form.
U.S. Pat. No. 3,744,779 to Perry describes the recovery of metal values in scrap materials by heating sufficiently to destructively distill organic compounds and recover melted metal in the absence of oxygen.
U.S. Pat. No. 3,770,419 to Brown describes a pyrolysis system which produces molten metal from refuse containing metal.
U.S. Pat. No. 4,230,043 to Deardorff describes a system of destroying polybrominated biphenyls by exposure at 3000.degree. F. to iron by-product from steel making, a mineral acid and a pulverized reducing metal.
U.S. Pat. No. 5,085,738 to Harris describes the destruction of organic wastes by thermal conversion in a pool of molten lead in the absence of oxygen.
U.S. Pat. No. 3,890,908 to von Klenck et al describes pyrolytic destruction of waste by passage through a bath of molten metal or glass in the absence of air.
U.S. Pat. No. 4,246,255 to Grantham et al describes the disposal of PCBs by feed PCB's and a source of oxygen into a molten salt bath.
The field of pyrolytic destruction of wastes is one of active research and this list is intended merely to represent key variations in the technologies which might be applied as a pyrometallurgical step. None of the foregoing teaches or suggests why or how a powder of an alloy of Co, Mn and C would provide an effective medium for the reconstitution of a catalyst comprising a mixture of Co and Mn acetates.
Providing a feed stream of the organic residue under appropriate process conditions to one of the aforementioned pyrometallurgical processes, will result in two product streams and a slag: (i) a gas phase comprising carbon monoxide, hydrogen and bromine values; (ii) molten metal containing cobalt, manganese and carbon; and, depending on the composition of the feed, the slag composition consisting essentially of metal oxides other than those of Co and Mn, although a substantial amount of Mn may be directed to the slag, as specifically described in the parent '194 application. Granted that the molten metal generated is in the form of a Co/Mn or Co/Mn/C alloy, there is no motivation to generate a powder, then pursue digestion of the powder in acetic acid, or, a quench tower or scrubber recycle stream (hereafter "quench stream") obtained by absorbing evolved gases in aqueous acetic acid, or mixtures of acetic acid and the quench stream, unless one's goal is to re-manufacture the Mid-Century catalyst. Even so, the prior art does not teach how such digestion may be satisfactorily accomplished.
The preparation of catalyst feed solutions for the Mid-Century process is currently performed by any of several methods. Co and Mn metals are separately digested in glacial acetic acid, or concentrated aqueous acetic acid, and added to the reactor separately to allow precise control of the ratio of the Co and Mn acetates. Alternatively, cobalt or manganese acetate can be prepared by digestion of the respective metal hydroxides. These hydroxides are often prepared by digesting the metal in a strong acid such as nitric acid, precipitating by treatment with caustic, washing, and collecting the insoluble hydroxide (see U.S. Pat. No. 1,637,281 to Schatz).
Preparation of cobalt acetate directly from metal is problematic, requiring the use of large excess of metal. The prior art favors the use of promoters such as bromide salts of compatible metals or elevated pressures and the use of oxygen to obtain effective digestion. When Co is in the form of powder, introduction of oxygen into the vessel to obtain a practical rate of digestion in 50% acetic acid, still allows only 86% of the cobalt to be digested after 2 hr. To get substantially complete conversion it is necessary to add an activator (see U.S. Pat. No. 3,133,942 to Hahl, Examples 4 et seq). One would therefore not expect that cobalt would be substantially completely digested in the absence of added oxygen. In order to process cobalt in the form of electrolytic chip (pieces about 1/8" by 11/4"), U.S. Pat. No. 4,921,986 to Fox teaches the use of elevated pressures to increase the oxygen concentration in solution.
The alloy obtained in the pyrometallurgical system of this invention has as its main components: cobalt, manganese and carbon, the amount of carbon remaining depending upon treatment of molten alloy. In an effective process for reconstituting a Co/Mn/C catalyst in a reasonable period of time, less than 24 hrs, it is critical that substantially complete digestion of the metals in this alloy be obtained. By "substantially complete digestion" I refer to the digestion of at least 90% of the Co/Mn metals. Partial digestion, unless proportional, is unsuited for the reconstitution of this catalyst because changing the Co/Mn ratio from that received in the waste, would require readjusting the ratio. The problem of disposing of, or otherwise treating or re-processing the undigested material would remain. The prior art does not teach the digestion of a Co/Mn/C alloy in acetic acid. There is no suggestion in the prior art that an alloy of Co/Mn/C be deliberately converted to a powder for the purpose of digestion in non-alcoholic glacial acetic acid, non-alcoholic solutions of acetic acid, or a mixture of non-alcoholic acetic acid and the quench stream; and, no suggestion that the powder be in a size range smaller than 5000 .mu.m, preferably in the range from about 5 .mu.m to 1000 .mu.m, most preferably in the range from 45 .mu.m to 500 .mu.m, so that it will lend itself to substantially complete digestion of the metals in the aforesaid digesting liquids even at ambient pressure, making superatmospheric digestion in the aforesaid solutions unnecessary.
It is well-known that Mn powder is readily digested in hot acetic acid; and powdered Co is partially digested in 80% acetic acid (20% water) but leaves a heel even at reflux at ambient pressure. Typically, the alloy herein contains carbon present in an amount smaller than either the Co or Mn but greater than 0.5%. The amount of carbon held in the alloy is limited by the saturation limit. This limit may be as much as 7% based on the total weight of the alloy, and depends on the composition of the alloy (see CR Acatt Sc. Paris, vol 264 pg 281-284, Jan. 16, 1967). One would expect the difficulty of digesting an alloy with the carbon to be increased greatly, since it is well known that as little as 0.03% carbon in stainless steel helps resist intergranular corrosion (see Corrosion Resistance Tables, by Schweitzer, Philip A., pg vii, Marcel Dekker Inc.). It is accepted that corrosion is digestion occurring at a slow rate. Further, knowing that it is not possible to predict the rate of corrosion of an alloy from data relating to the rates of corrosion of the alloy's individual components, one cannot predict how easily an alloy of Co and Mn will be substantially completely digested in the aforesaid digesting liquids, if the alloy is digested at all. Whatever the rate of corrosion of the alloy of Co/Mn, the presence of carbon in the alloy would simply complicate the task of predicting the rate.
G. Arrivaut (Proces-Verbaux, 1905 pp 107-114), describes the prepartion and digestion of several Co/Mn alloys. The alloys containing from 60-85% Mn digest completely in ammonium chloride, hydrochloric acid, sulfuric acid, nitric acid and hydrofluoric acid. In alcoholic solutions of acetic acid, only partial digestion occurs, and the unreacted metal is greatly enriched in cobalt. Arrivaut does not teach the digestion of Co/Mn alloy in acetic acid solutions other than the dilute alcohol system. Arrivaut also notes that the residual material is a pyrophoric powder. (see Comprehensive Treatise on Inorganic & Theoretical Chemistry, by Mellor, J. W. Vol. XIV, pg 544, Longmans, Green & Co., New York).
With particular respect to the quench stream obtained by absorbing evolved gases from the reaction zone of the Mid-Century process in which the catalyst is used, the absorbent stream may be aqueous acetic acid, or a mixture of acetic acid and HBr, and/or MnBr.sub.2, and such other products of reaction which are absorbed during recirculation of the stream through the absorption zone. Since the composition of the stream varies in both acetic acid and HBr, as well as other components, it's effect on the alloy in powder form, is not predictable.
The process of this invention is uniquely well suited to recover essentially all of the Co, Mn and Br values in both of these residue streams. By "essentially all" is meant that in excess of 90%, typically in excess of 95%, and preferably in excess of 99% of the components may be recovered. The process is operated to re-manufacture a catalyst at the same ratio as the incoming residue. By "incoming residue" is meant a single stream from a given Mid-Century plant, or the combination of multiple streams from plants producing the same or different product, or streams of earthy residues, or streams of earthy residues and plant residue streams.
A further advantage of this invention is that much equipment already existing in a facility for the recovery of Co and Mn values from flyash may be used to remanufacture catalyst from the recovered Co/Mn/C alloy thus decreasing costs.